The present work investigates effect of nitrogen pressure on composition and mechanical properties of superhard nc-TiAlN/a-Si3N4 nanocomposite coatings deposited by cathodic arc PVD process. As the nitrogen pressure increases, at.% ratio of (Al+Si)/Ti, initially, increases to a maximum and thereafter, it decreases while the nitrogen content follows the reverse trend. Hardness is influenced by chemical composition and crystallite size of TiAlN phase. Maximum average hardness of 37 GPa is achieved when at.% ratio of Al(+Si)/Ti or N/(Ti+Al+Si) approaches 1.0 and decrease in scratch adhesion strength is attributed to the defects caused by nitrogen deficiency.